1. Field of the Invention
The present invention relates to a method of producing an L-amino acid using a bacterium, and more particularly, to a method of producing an L-amino acid such as L-lysine, L-threonine, and L-glutamic acid. L-lysine and L-threonine are useful as additives in animal feeds, components of health food, amino acid infusions, and the like. L-glutamic acid is useful as a food seasoning.
2. Brief Description of the Related Art
L-amino acids have been industrially produced by fermentation using bacteria belonging to the genus Brevibacterium, Corynebacterium, Escherichia, or the like. Methods of producing L-lysine are described in EP 0643135 B, EP 0733712 B, EP 1477565 A, EP 0796912 A, EP 0837134 A, WO 01/53459, EP 1170376 A, and WO 2005/010175. In these methods, bacterial strains are used which are isolated from nature or artificial mutants thereof, as well as bacterial strains which have been modified to enhance the activity of an L-amino acid biosynthetic enzyme by recombinant DNA techniques.
Methods are known for improving L-amino acid-producing ability, and include modifying the uptake or export of L-amino acids in and out of cells. A known method of enhancing L-amino acid export is to produce L-lysine (WO 97/23597) or L-arginine (US 2003-0113899) using a bacterial strain belonging to the genus Corynebacterium which has been modified so that expression of an L-lysine/L-arginine export gene (LysE) is enhanced. In addition, methods have been reported of producing an L-amino acid using a bacterium belonging to the Enterobacteriaceae family which has been modified so that expression is enhanced of the rhtA gene, rhtB gene, and rhtC gene (EP 1013765 A), yfiK gene or yahN gene (EP 1016710 A), ybjE gene (WO 2005/073390), or yhfk gene (WO 2005/085419). Each of these genes have been suggested to be involved in L-amino acid export.
It is also known that enhancing the expression of an uptake gene for a sugar improves the L-amino acid-producing ability. This is because sugars typically function as a substrate during fermentation. Examples of such methods include producing an L-amino acid using an Escherichia bacterium modified to have enhanced expression of the ptsG gene (WO 03/04670), ptsH gene, ptsI gene, or crr gene (WO 03/04674).
The fepA gene and the fecA gene each encode a membrane protein which is known as an iron transporter, while the tonB gene encodes a protein that regulates the activity of the iron transporter (J. Bacteriol. 1990; 172(5): 2736-46, J. Bacteriol, 2003, vol. 185, No. 6, p 1870-1885, Mol. Microbiol. 2005; 58(5): 1226-1237, J. Bacteriol 2001, vol. 183, No. 20, p 5885-5895). However, there have been no reports that enhancing the activities of these gene products can be effective for L-amino acid production.